Implement system call for bulk retrieveing of opened descriptorsin binary form.

Some daemons could use it to reliably close file descriptorsbefore starting. Currently they close everything upto some numberwhich formally is not reliable. Other natural users are lsof(1) and CRIU(although lsof does so much in /proc that the effect is thoroughly buried).

Once again, /proc, the only way to learn anything about file descriptorsmay not be available.